1. Field of the Invention
The present invention relates to a one-way rotational transfer mechanism having a rotary input shaft and a rotary output shaft which are concentrically arranged, wherein rotation of the rotary input shaft is transferred to the rotary output shaft when the rotary input shaft is rotated, but rotation of the rotary output shaft is not transferred to the rotary input shaft when the rotary output shaft is rotated. The present invention further relates to a lens barrel which incorporates such an one-way rotational transfer.
2. Description of the Related Art
Among conventional one-way rotational transfer mechanisms having a rotary input shaft and a hollow-cylindrical rotary output shaft positioned radially outside the rotary input shaft, wherein the rotary input shaft is driven by motor to transfer rotation of the rotary input shaft to the hollow-cylindrical rotary output shaft, a one-way rotational transfer mechanism which prevents the motor from being rotated by rotation of the rotary output shaft (i.e., prevents the rotary input shaft from being rotated by rotation of the rotary output shaft) when the rotary output shaft is rotated has been proposed in, e.g., Japanese laid-open patent publication H08-177878 and Japanese laid-open patent publication H11-202181.
Among conventional lens barrels available for both autofocus (AF) and manual focus (MF), a lens barrel including a stationary ring, a movable ring, a distance adjustment ring, and an AF motor and a manual focus ring is known in the art, wherein the movable ring is concentrically arranged with the stationary ring, supports a focusing lens group and is movable forward and rearward along an optical axis, the distance adjustment ring is concentrically arranged with the stationary ring and rotates on an axis of the distance adjustment ring to move the movable ring forward and rearward along the optical axis, the AF motor drives the distance adjustment ring, and the manual focus ring is manually rotated about its axis to rotate the distance adjustment ring.
Furthermore, among the aforementioned conventional lens barrels usable for both autofocus (AF) and manual focus (MF), a lens barrel which incorporates a one-way rotational transfer mechanism between the distance adjustment ring and an AF motor, wherein a torque of the distance adjustment ring rotated by a rotation of the manual focus ring is not transferred to the AF motor when the manual focus ring is manually rotated even though a driving force of the AF motor is transferred to the distance adjustment ring when the AF motor is driven, is known in the art.
This type of lens barrel is disclosed in e.g., the aforementioned Japanese laid-open patent publication H08-177878. In this type of lens barrel, a manual focusing operation can be carried out even in AF mode with an AF switch remained ON.
However, in the lens barrel disclosed in Japanese laid-open patent publication H8-177878, a driving force of the AF motor is not smoothly transferred to the distance adjustment ring when the AF motor is driven because of heavy backlash which occurs upon rotation of the rotary input shaft (upon startup of the motor). Moreover, the torque transferred to the distance adjustment ring becomes small when driven at a slow rotational speed because the torque transferred to the distance adjustment ring is proportional to the rotational speed of the rotary input shaft.
In the one-way rotational transfer mechanism disclosed in the aforementioned Japanese patent publication H11-202181, the structure thereof is complicated. Moreover, it is difficult to increase the number of rollers serving as torque transfer members due to the complicated structure of the one-way rotational transfer mechanism, and accordingly, the torque which is transferred from the rotary input shaft to the hollow-cylindrical rotary output shaft when the rotary input shaft is driven to rotate cannot be increased very much.
Furthermore, in the lens barrel disclosed aforementioned Japanese laid-open patent publication H08-177878, there is a possibility of the one-way rotational transfer mechanism becoming incapable of securely transferring torque from the AF motor to the distance adjustment ring to thereby make it impossible to perform an autofocusing operation when the AF motor is driven.